1. Field of the Invention
The present invention relates to a laser control unit, a laser control circuit and a laser-power adjustment method which are capable of controlling the recording optical power of a semiconductor laser which emits a beam of light so that a mark area which corresponds to a data signal can be recorded on an optical disk.
2. Description of the Related Art
In a conventional laser control unit for an optical disk, there is known the following method (e.g., refer to Patent Document 1). In that method, when data is additionally written, feed-forward control of the power value of a recording optical pulse is executed at high speed. Thereby, a beam of light is emitted from a semiconductor laser, at precise power from the head of a sector. FIG. 27 shows a conventional laser control method which is described in Patent Document 1.
First, as shown by detection division in FIG. 27(A) and a waveform of emitted light in FIG. 27(B), an optical pulse is generated which is made up of a constant emission of bottom power, a constant emission of bias power, and a multi-pulse emission. This optical pulse is received by a photo-detector and is converted into an electric current. Then, it is converted into a voltage in a current-voltage conversion circuit, so that a waveform of received light is obtained as shown in FIG. 27(C). Based on this received-light waveform, there are obtained a bottom DC value Bdc, a bias DC value E, and in addition, an average value M of the multi-pulse segment.
Then, using the obtained bottom DC value Bdc, average value M of the multi-pulse segment and a duty d of the multi-pulse segment, an arithmetic processor calculates a peak value P of the multi-pulse segment which is an unknown numerical value in the following expression (56).P={M−Bdc×(1−d)}/d  (56)
The obtained peak value P, bias value E and bottom value Bdc are compared with set values Pref, Eref and Bref, respectively. Thereby, the arithmetic operations of differential values ΔP, ΔE and ΔB are carried out, respectively.ΔP=P−Pref  (57)ΔE=E−Eref  (58)ΔB=B−Bref  (59)
The differential values ΔP, ΔE and ΔB which have been obtained from these arithmetic operations are added to the peak value P, bias DC value E and bottom DC value Bdc of the present power, respectively, so that they are converted into an electric-current value which is sent to a semiconductor laser. Thereby, the differential values ΔP, ΔE and ΔB are controlled so as to become zero.
If an electric-current conversion coefficient is K, a peak electric-current value is Ip, a bias electric-current value is Ie, and a bottom electric-current value is Ib, then those values are calculated, using the following expressions (60) to (62), respectively.Ip=K(P+ΔP)  (60)Ib=K(B+ΔB)  (61)Ie=K(E+ΔE)  (62)
Using such a method, control is executed so that immediately after data has been recorded, the recording power of a semiconductor laser can be settled at high speed to a target power.
Herein, Patent Document 1 is Japanese Unexamined Patent Publication No. 2002-203320 specification.
However, the above described conventional configuration has the following disadvantage. A pulse-current source which drives a laser has a characteristic which may produce a discrepancy between the duty of the multi-pulse segment and the set duty d. Then, this discrepancy depends upon the current value of the pulse-current source which drives a laser. In the future, it is expected that the speed of processing will be several times as high as the present one, and that the pulse-current value may go up. In that case, such a discrepancy of the duty d becomes larger. While the duty of an optical pulse is shifted from the set duty d, an arithmetic processor calculates a peak value, using the set duty d as it is. This may produce an error in the peak value. When the speed of processing becomes several times as high in the future, an unacceptable error may be produced.